This proposal is concerned with the selectivity problems encountered in the synthetic application of hydrogen reactions like face- and cis-selective hydrogenation, cis-selective semi-hydrogenation, selective hydrogenation of polyenes, and selective hydrogenation of functionalized olefins. The surface structure sensitivity and demanding nature of these hydrogenation reactions and their associated side reactions will be identified. A Pd-membrane reactor will be used to differentiate between surface hydrogen and sub-surface hydrogen as active species in hydrogenation reactions. The mechanistic insight obtained will guide the development of new experimental techniques for more selective hydrogenation reactions. Thin film materials of ordered surface structure will be prepared, characterized, and tested as new prototypes of selective hydrogenation catalysts.